R723, a selective JAK2 inhibitor, effectively treats JAK2V617F-induced murine myeloproliferative neoplasm.

The activating mutations in JAK2 (including JAK2V617F) that have been described in patients with myeloproliferative neoplasms (MPNs) are linked directly to MPN pathogenesis. We developed R723, an orally bioavailable small molecule that inhibits JAK2 activity in vitro by 50% at a concentration of 2nM, while having minimal effects on JAK3, TYK2, and JAK1 activity. R723 inhibited cytokine-independent CFU-E growth and constitutive activation of STAT5 in primary hematopoietic cells expressing JAK2V617F. In an anemia mouse model induced by phenylhydrazine, R723 inhibited erythropoiesis. In a leukemia mouse model using Ba/F3 cells expressing JAK2V617F, R723 treatment prolonged survival and decreased tumor burden. In V617F-transgenic mice that closely mimic human primary myelofibrosis, R723 treatment improved survival, hepatosplenomegaly, leukocytosis, and thrombocytosis. R723 preferentially targeted the JAK2-dependent pathway rather than the JAK1- and JAK3-dependent pathways in vivo, and its effects on T and B lymphocytes were mild compared with its effects on myeloid cells. Our preclinical data indicate that R723 has a favorable safety profile and the potential to become an efficacious treatment for patients with JAK2V617F-positive MPNs.

Tyrosine kinase 2 interacts with the proapoptotic protein Siva-1 and augments its apoptotic functions.

Siva-1 is a molecule that has the potential to induce both extrinsic (receptor-mediated) and intrinsic (non-receptor-mediated) apoptosis. Siva-1 binds to CD27, a member of the tumor necrosis factor receptor (TNFR) family, Abl-related gene (ARG), and BCL-X(L), and these partner molecules reportedly enhance the apoptotic properties of Siva-1. In this study, we show that Siva-1 also interacts with a member of the Jak family protein kinases, tyrosine kinase 2 (Tyk2). Siva-1 bound to Tyk2 via its N-terminal region, and Tyk2 phosphorylated Siva-1 at tyrosines 53 and 162. In murine pro-B cells, Ba/F3 cells, expression of Tyk2 augmented Siva-1-induced apoptosis. This augmentation of Siva-1-induced apoptosis was retained regardless of the phosphorylation of Siva-1, but was almost completely prevented by the abrogation of the Tyk2-Siva-1 association. These findings indicate that the interaction between Siva-1 and Tyk2 directly augments the apoptotic activity of Siva-1. Our novel observations suggest that Siva-1 forms a functional complex with Tyk2 and participates in the transduction of signals that inhibit B lymphocyte growth.

JAK2 V617F uses distinct signalling pathways to induce cell proliferation and neutrophil activation.

The acquired JAK2 V617F mutation is observed in the majority of patients with BCR-ABL1 negative chronic myeloproliferative neoplasms (MPN). BCR-ABL1 negative MPN displays myeloproliferation with an elevated leucocyte alkaline phosphatase (LAP) activity, a neutrophil activation marker. We tried to separate the downstream signalling of JAK2 V617F to stimulate myeloproliferation and LAP activity. NB4, a myeloid lineage cell line, was transduced with Jak2 V617F mutation or wild-type Jak2. We found that Jak2 V617F mutation, but not wild-type Jak2 enhanced LAP expression in NB4-derived neutrophils and proliferation of NB4 cells. JAK2 V617F induces constitutive phosphorylation of STAT3 and STAT5, and uses signalling targets such as Ras/MEK/ERK and PI3K/Akt pathways. By using MEK1/2 inhibitor U0126, PI3K inhibitor LY294002, and STAT3 or STAT5 siRNAs, JAK2 V617F was found to specifically use the STAT3 pathway to enhance LAP expression, while STAT5, Ras/MEK/ERK and PI3K/Akt, but not STAT3 pathways, were able to stimulate cell proliferation. These data strongly suggest that JAK2 V617F uses distinct signalling pathways to induce typical pathological features of MPN, such as high LAP activity and enhanced cell proliferation.

Chronic thrombopoietin overexpression induces mesangioproliferative glomerulopathy in mice.

We previously reported that mice transgenic (Tg) for thrombopoietin (TPO) developed progressive fibrosis and osteosclerosis of the bone marrow. Here, we show that TPO-overexpressing mice also exhibited notable histological changes in the kidneys, including an increased number of mesangial cells, expansion of the mesangial matrix in the glomerulus, and atrophy of the renal tubuli. Plasma transforming growth factor (TGF)-beta1 and platelet-derived growth factor (PDGF)-BB, which could induce mesangioproliferative responses in glomeruli, were both elevated in TPO Tg mice, even though TPO itself has no effect on mesangial cells due to their lack of c-Mpl. The mesangial proliferative change in TPO Tg mice was thought to be induced by the elevation of these cytokines. In conclusion, our finding that TPO-overexpressing mice developed mesangioproliferative glomerulopathy might represent an undesirable effect of chronically elevated TPO in vivo, which should be taken into consideration before new TPO-like growth factors become available in clinical practice.